This invention relates generally to methods and apparatus for enhancing fabric filter performance, and more particularly to electrically charged fabric filter performance.
At least one known ESFF (electrostatic fabric filter) configuration (e.g., Max-9™, available from BHA Group, Inc., Kansas City, Mo.) offers a variety of configurations that can be adapted to existing plants. These filter configurations utilize a high air-to-cloth ratio, so it is relatively easy to fit into a cramped plant environment. These filter configurations are also easily retro-fitted into existing environments.
In some known configurations, the polarity of a charging field on an ESFF bag house is reversed to facilitate cleaning of the filter bags. A sufficient amount of cleaning air at a suitable pressure enters the outlet of the filter bag or element. The flow of air reverses gas flow through the bag and shocks the cloth, resulting in particulate matter being dislodged from the surface of the fabric filter.
In ESFF, all particles entering the bag house are charged with a negative corona, causing all particles to migrate to the positive end of the filter bag. In many known configurations, air pulse cleaning is used. Air pulse cleaning, however, is not necessarily completely effective in removing all charged dust particles from a filter bag. Some particles tend to cling to the filter as a result of electrical charges.
The common polarity of dust particles accumulating on the surface of the filter bag can result in a porous cake. “Like” charged particles repel one another on the filter bag, creating interstitial passages through which gas can freely flow. As the dust layer increases in thickness the pressure drop increases. When pressure drop reaches a pre-set upper limit, a pulse cleaning cycle must be initiated by directing a blast of compressed air into the throat of the filter bag where clean gas exits. The compressed air reverses flow of gas through the inside of the filter causing dust to be force off the external surface of the bag. This shock causes the dust layer to diminish in thickness reducing pressure drop. The process of pulse cleaning occurs on a cyclical basis to maintain system pressure drop within an acceptable range.
The cleaning process introduces fatigue and wear in the filter bag. As a result, bag life is dependent on the number of cleaning cycles experienced and the pressure of the air utilized.
The high voltages that are applied to filters also carries with it a potential for sparking. Sparking can burn holes in filter bags, from which may result immediate increases in dust emissions. Fabric filters are designed to minimize the propensity for sparks to occur, but in known filter configurations, the waveform from the transformer rectifier power supply is negative pulsed DC, which produces a series of half cycle waves. To achieve an average voltage of 35 kV, which is used in many ESFF configurations, a peak voltage of about 42 kV to 48 kV must be realized. It is thus necessary to design the filter to withstand at least this high peak voltage, plus an additional margin of safety.